Your authentication checks should be in a library:
The is an excerpt from a basic codigniter authentcation script:
class Site_sentry 
{
    function Site_sentry()
    {
        $this->obj =& get_instance();
    }
function is_logged_in()
{
    if ($this->obj->session) 
    {
        if ($this->obj->session->userdata('session_logged_in'))
        {
            return TRUE;
        }
        else
        {
            return FALSE;
        }
    }
    else
    {
        return FALSE;
    }
} 
   function login_routine()
   {
     //do login here (enter into session)
   }
}
This library is stored in application/libraries under a filename named after its class with the .php suffix.
Then you can either add this to your autoload config file application/conig/config.php:
$autoload['libraries'] = array('database', 'site_sentry', 'session');
or load it manually in each controller:
$this->load->library('Site_sentry);
Then you can check your session from within controllers, like so:
 class Class extends Controller{
    function Class()
    {   
       parent::Controller();
       if( $this->site_sentry->is_logged_in() == FALSE){
            redirect('managerlogin/');
        }
    }
   }
Also check this documentation page http://codeigniter.com/user_guide/libraries/sessions.html; of particular interest is the storing the session into the database section.